N567WN – BOEING 737-7CT – 2024-04-03 NTSB Accident Report

HISTORY OF FLIGHTOn April 3, 2024, about 0820 eastern daylight time (EDT), Southwest Airlines (SWA) flight 4273, a Boeing 737-700 airplane, experienced turbulence while enroute between Louis Armstrong New Orleans International Airport (MSY), New Orleans, Louisiana, and Orlando International Airport (MCO), Orlando, Florida. Of the 5 crew and 140 passengers onboard, one FA and one passenger sustained serious injuries, and one FA sustained minor injuries. The aircraft was not damaged. The flight was operated under the provisions of Title 14 Code of Federal Regulations Part 121 as a scheduled passenger flight.

According to the captain, the incident flight was the first flight of the day and was scheduled to depart at 0740 central daylight time (CDT). Upon arrival at the airplane, the captain coordinated with the gate agent, then assisted the first officer (FO) with the preflight checklists while the FAs prepared the cabin. The FO then went to the ramp to complete the walkaround while the captain briefed the FAs, reviewing the flight route, weather, flight time, and information about special passenger needs. The crew collectively decided that the FAs would evaluate the flight conditions at 10,000 ft and start service when it was smooth. The captain informed the FAs that if they experienced turbulence, they should take their jumpseats.

The captain and FO, who was the pilot flying, conducted a preflight briefing and reviewed the available information, including the planned destination alternate and enroute weather. The flight departed MSY about 0755 CDT and encountered light chop through the climb. The crew reported that the fasten seat belt sign remained on for the entire flight. The crew stated they leveled at flight level (FL)370, above the cloud tops, and continued to monitor the enroute weather on the airplane radar and the captain’s electronic flight bag weather apps. About 40 minutes later, while approaching an en route waypoint, the airplane encountered light turbulence and the flight crew coordinated with ATC to deviate around the weather. ATC initially suggested a right deviation, to the south, but the information available to the flight crew indicated that it was not the best route.

The flight was handed off to another controller, who gave the flight crew the option to descend to FL350 and deviate to the right, or deviate left at their current altitude. The flight crew elected to remain at their altitude, above the cloud tops, and deviated to the left. After deviating left of course, the crew began to turn right, back toward the waypoint, and observed a rapidly developing cloud. The FO suggested they continue to turn to the right to avoid the buildup and the captain agreed. The captain used the public address system to request the FAs to take their seats.

The captain informed ATC of the deviation and the FO reduced the airspeed in anticipation of turbulence. The flight crew stated that they penetrated the buildup and encountered about 10 seconds of severe turbulence. During the encounter, the airplane experienced about 30° of bank, airspeed fluctuations of 15-20 kts, and altitude deviations of +200 ft to -100 ft. The flight crew reported hearing the aural airspeed limitation alert.

According to the A position FA, while completing cabin service, he heard the captain’s announcement and was able to make it to the forward jumpseat at the onset of the turbulence. He stated that the turbulence was initially moderate and he jostled around, but once he was able to secure the seat belt, the turbulence increased to severe. From his jumpseat he observed the C position FA sit in the aisle during the turbulence encounter. After the turbulence encounter, the A FA walked through the cabin to inspect for damage and check for injuries. The C FA, in the aisle, sustained minor injuries, but the B position FA and a passenger were lying on the floor in the vicinity of the aft galley. The B FA had bumped her head, potentially fractured her right arm, and had pain in her right hip. The passenger had pain in her neck and back. The A FA contacted the flight crew and reported the damage and injuries.

The flight crew elected to declare a medical emergency and initiated a diversion to Tampa International Airport (TPA), Tampa, Florida. The FO continued pilot flying duties, and the captain coordinated the diversion with company dispatch.

While preparing for landing, the C FA took the forward jumpseat and the A FA sat on the floor in the aft galley area with the injured B FA and passenger. The approach and landing at TPA were without incident. The airplane parked at gate 32 and paramedics met the flight. The captain made an announcement for passengers to remain seated, and the paramedics boarded the airplane. Once all passengers were deplaned, paramedics used a provisioning truck to lower the two injured people to the ramp level, from which they were transferred to ambulances and transported to the hospital. AIRCRAFT INFORMATIONThe airplane was a Boeing 737-700, manufacturing Serial Number 32747. The airplane was manufactured in 2002, delivered to Southwest Airlines in 2014, and held a transport category airworthiness certificate. The airplane was configured with 2 flight crew seats, 3 flight attendant seats, and 143 passenger seats. At the time of the event, there were no deferrals from the minimum equipment list.

Figure 1. Accident Airplane, N567WN (Source: planespotters.net) METEOROLOGICAL INFORMATIONA review of the NWS large-scale migratory weather systems at the time of the encounter with turbulence indicated that a strong cold front supported the development of a squall line that extended from the Florida Panhandle into the Gulf of America. The upper air charts depicted an upper-level low pressure system over Lake Michigan with a long wave trough extending southwestward into Texas. Winds over the area were from the west at nearly 100 kts over the turbulence encounter location, at the airplane’s cruising level of FL370. A strong 150- to 170-kt jet stream extended immediately north over eastern Texas and Louisiana and into Alabama and Georgia, with a likelihood of occasional moderate turbulence.

The Meteorological Aerodrome Reports for Apalachicola Regional Airport (KAAF), Apalachicola, Florida, about 110 miles north-northeast of the event location, reported a period of thunderstorms and heavy rain with outflow winds just below the severe storm limit of 50 kts prior to the accident, with rainfall of 1.33 inches. This coincided with the passage of the squall line over the area.

The High-Resolution Raid Refresh sounding indicated that the atmosphere was moist with a precipitable water content of 2.16 inches, a lifted index of -3.1 with a positive Convective Available Potential Energy of 1,079 Joules per kilogram, and maximum upward vertical motions up to 96 kts or 9,055 ft per minute. Estimated cumulonimbus cloud tops to FL410 were expected based on the sounding, while real-time satellite cloud tops were observed from FL380 to FL460.

The Eglin Air Force Base (KEVX) Weather Surveillance Radar-1988 Doppler (WSR-88D) and NWS composite radar imagery depicted echoes of 56 decibels of reflectivity (dBZ) under the airplane’s flight track, with the higher 1.8° base reflectivity imagery depicting echoes up of 22.5 dBZ at the accident airplane’s cruising altitude. Airborne weather radar typically displays echoes greater than 20 to 30 dBZ as light intensity echoes. Therefore, echoes were likely just beginning to appear on airborne radar near the time of the encounter. The KEVX WSR-88D long-range enhanced echo tops product depicted tops in the range of FL400 during the period.

Figure 2. Weather radar image with airplane track and location overlayed at the time of the turbulence encounter.

The FAA’s Corridor Integrated Weather System and Storm Prediction for Aviation and Offshore Precipitation Capability imagery were also examined and depicted extreme intensity echoes moving across the area with significant cloud-to-ground lightning. While there was no significant difference noted between the weather displays in the vicinity of the turbulence location within the ZJX airspace, several of the routes further south of the accident flight would have only been viewable using the Offshore Precipitation Capability data due to range limitations.

The ATC facilities weather services unit also provided a morning briefing to all oncoming controllers advising them of the broken line of thunderstorms that was expected to cross their airspace during the period.

A review of the pilot reports issued surrounding the period of the turbulence encounter indicated no specific reports along the route of flight of any significant turbulence prior to the accident flight.

The NWS convective forecast products provided some potential warning of thunderstorms across the Gulf of America in the vicinity of the turbulence event, which implied the potential for severe turbulence. While the NWS Storm Prediction Center convective outlook discussed the potential for severe thunderstorms over the United States, it provided no information or details of the convection offshore. The Traffic Flow Management Convective Forecast indicated a medium coverage of storms with tops above FL400 from southeast Georgia and the Florida panhandle to several hundred miles into the Gulf of America for the 2-, 4-, and 6-hour periods.

The NWS Aviation Weather Center’s graphic area forecast indicated scattered coverage of thunderstorms during the period. The NWS Graphic Turbulence Guidance (GTG) product predicted light to moderate turbulence associated with the front’s position; however, it did not include the convective potential. The current National Center for Atmospheric Research (NCAR) Graphic Turbulence Guidance-Nowcast (GTG-N) 1- and 2-hour testbed products also failed to depict the potential convective turbulence. The NCAR GTG-N v1 with lightning data and a finer domain, however, dem...